1. Field of the Invention
The present invention relates to a liquid crystal device and, more particularly, to a liquid crystal device in which bulkheads maintain a desired clearance between a pair of substrates retaining a liquid crystal therebetween.
2. Related Background Art
Conventionally, CRTs are known as displays that have most commonly been used heretofore, and the CRTs are now widely used as monitors for output of moving picture of TV, VTR, or the like, or for personal computers. However, the CRTs have such characteristics as to degrade the visibility by flicker, stripes due to insufficient resolution, etc. and deteriorate a phosphor by image persistence in the case of still images. Further, they have a large volume behind the screen because of their structure, which impedes space saving at offices and homes.
A solution to such imperfections of the CRTs was liquid crystal apparatus and among the known liquid crystal apparatus was one provided with a liquid crystal device using a twisted nematic (TN) liquid crystal, for example, as described in M. Schadt and W. Helfrich, xe2x80x9cApplied Physics Letters Vol. 18, No. 4, p127-128 (Feb. 15, 1971).xe2x80x9d
One of such liquid crystal devices using the TN liquid crystal was of the passive matrix type holding superiority in cost, but the liquid crystal devices of this type had the problem that crosstalk occurred during time-sharing addressing in matrix electrode structure of a high pixel density, and thus had a limit to the number of pixels.
On the other hand, the liquid crystal devices called TFT devices, different from the passive matrix type devices, have been developed in recent years. Since a transistor is fabricated at every pixel, these TFT liquid crystal devices solve the problems of crosstalk and slow response speed on one hand but have the following drawbacks on the other hand, however: it becomes harder to fabricate the liquid crystal device without defective pixels as the area increases, and, even if possible, the cost becomes enormous.
For overcoming the drawbacks of the conventional liquid crystal devices as described above, Clark and Lagerwall proposed the liquid crystal device of the type utilizing the refractive index anisotropy of ferroelectric liquid crystal molecule and controlling transmitted light rays by combination with a polarizing element (Japanese Patent Application Laid-Open No. 56-107216, U.S. Pat. No. 4,367,924, and so on).
In general, this ferroelectric liquid crystal (FLC) has a chiral smectic C phase (SmC*) or H phase (SmH*) in a specific temperature region and in this condition, it has such a property that it takes either of a first optically stable state and a second optically stable state in response to an applied electric field and it maintains either state in the absence of application of an electric field, i.e., bistable memory nature. Moreover, it undergoes inversion switching because of spontaneous polarization and thus demonstrates a very fast response speed. Further, it is also excellent in viewing angle characteristics and is thus suitable, particularly, for high speed, high definition, and large screen display devices.
Incidentally, such ferroelectric liquid crystal devices in an initial orientation stage are in a state in which liquid crystal molecules oriented in a first stable state and liquid crystal molecules oriented in a second stable state are mixed in a domain. Namely, since the chiral smectic liquid crystal in the bistable state has almost equivalent energy levels of orientation regulating force to orient the liquid crystal molecules into the first stable state and orientation regulating force to orient the liquid crystal molecules into the second stable state, the liquid crystal molecules oriented in the first stable state and in the second stable state are mixed in each domain in the initial orientation stage, on the occasion of alignment under a condition of sufficiently thin alignment layers for the chiral smectic liquid crystal to demonstrate bistability.
On the other hand, among the ferroelectric liquid crystals is a xcfx84Vmin mode liquid crystal, which has negative dielectric anisotropy (xcex94{dot over (a)} less than 0) and positive biaxial dielectric anisotropy (xcex94{dot over (a)} greater than 0) and which exhibits a xcfx84Vmin characteristic, because the dielectrically anisotropic torque to stabilize the liquid crystal is greater than the reversing torque of ferroelectric liquid crystal.
The xcfx84Vmin characteristic is such a characteristic that the response speed of liquid crystal (xcfx84) a certain minimum (xcfx84Vmin) with increase in the applied voltage (V), and possession of this xcfx84Vmin characteristic makes it feasible to implement achievement of high luminance, high contrast, and high speed.
Liquid crystals demonstrating the antiferroelectric property are also known as the technology of constructing the display devices by making use of the refractive index anisotropy and spontaneous polarization of like liquid crystal molecules. Here the antiferroelectric liquid crystals (A-FLCs) generally have a chiral smectic CA phase (SmCA*) in a specific temperature region and in this condition, they have such a property that an average optically stable state is a direction normal to the smectic layer in the absence of the electric field but the average optically stable state is inclined from the direction normal to the layer in the presence of the electric field. In addition, the antiferroelectric liquid crystals also undergo switching because of coupling of spontaneous polarization with the electric field, thus exhibit very fast response speeds, and are expected to realize fast display devices.
Meanwhile, in order to uniformly drive the liquid crystal device employing the ferroelectric liquid crystal or the antiferroelectric liquid crystal, in the plane of the liquid crystal panel, it is necessary to keep glass substrates, which are an example of a pair of transparent substrates provided with transparent electrodes, uniform with a small fixed clearance (cell gap) between them.
The liquid crystal devices are normally constructed in such structure that the liquid crystal is filled in the small gap between two glass substrates and a voltage not less than a certain fixed threshold is applied between the transparent electrodes provided on the respective glass substrates to drive the liquid crystal. Because of this structure, if the gap between the glass substrates is nonuniform, different electric fields will be applied in plane to the liquid crystal panel, so as to cause in-plane (longitudinal) dispersion during driving of the liquid crystal.
Particularly, in use of the ferroelectric liquid crystal (FLC) or the antiferroelectric liquid crystal (A-FLC), the clearance between the pair of glass substrates needs to be as narrow as about 1 to 3 xcexcm, and production of the thin and uniform cell gap in plane is a hard technique while being also a very important constituent.
Methods of uniformly maintaining a pair of glass substrates with a small fixed clearance between are generally categorized into methods of placing spherical spacers between the substrates and methods of forming stripe bulkhead structures on at least one of a pair of substrates retaining the liquid crystal between, by employing flexible printing, photolithography, dry film, and so on.
FIG. 6 is a cross-sectional view of a liquid crystal device in which the cell gap is retained by use of the conventional spherical spacers. It is possible to form even a relatively narrow cell gap by use of the spherical spacers 50 as long as the spacers 50 can be made in uniform size. However, since a number of spacers 50 are scattered over one substrate 52 out of a pair of glass substrates 51, 52 in a liquid crystal device fabrication step, some spacers 50 are also placed within the pixel display areas. As a result, alignment defects occurred around the spacers 50 and posed a problem of failure in achieving satisfactory contrast of the liquid crystal device.
In order to maintain the cell gap, granular adhesive particles 53 are also scattered in addition to the spacers 50, as shown in FIG. 6, in certain cases. In such cases, alignment defects also occur around the adhesive particles 53 as around the spacers 50, and result in failure in achieving satisfactory contrast of the liquid crystal device.
On the other hand, FIG. 7 is a cross-sectional view of a liquid crystal device in which the cell gap is maintained by use of the conventional stripe bulkhead structures. When the stripe bulkhead structures 60 are employed, the bulkhead structures 60 are selectively laid in non-pixel areas of the liquid crystal device by the photolithography technology and thus no foreign matter is mixed in the pixel display areas, so as to cause few alignment defects.
Since it is also possible to provide the bulkheads themselves with a function of bonding the pair of upper and lower glass substrates 51, 52 to each other in addition to the function of controlling the cell gap, the number of alignment defects in the pixel display areas can be made much smaller than that in the case using the granular adhesive particles shown in FIG. 6.
In the case using the bulkhead structures 60, however, the liquid crystal 61 confined by the bulkhead structures 60 experiences volumetric shrinkage, as shown in FIG. 7, because of temperature change or phase change of the liquid crystal in process steps such as a filling step and the like. If the bulkhead structures 60 are unable to shrink similarly in response to the liquid crystal 61 by stress occurring between the substrates in accordance therewith, the interior of the liquid crystal will go into a negative pressure state and a void can appear and grow at a place or at two or more places in the liquid crystal, or a clearance can be made relative to the bulkhead surfaces.
The occurrence of a void in the liquid crystal device as described above will naturally result in making portions lacking the liquid crystal inside the liquid crystal device and during driving of the liquid crystal device such portions will not be driven, so as to remain as black patterns. As a result, the device will fail to accurately display letters, graphics, etc. and they can cause degradation of various characteristics such as lowering of contrast and the like.
Since the ferroelectric liquid crystals are smectic liquid crystals, they demonstrate large volumetric shrinkage, particularly, due to phase change, and thus to avoid the generation of a void is an important factor in fabrication of a panel with good characteristics.
The present invention has been accomplished in view of such circumstances and an object of the invention is to provide a liquid crystal device that can prevent the generation of a void part due to the phase change of liquid crystal.
According to a first aspect of the present invention, there is provided a liquid crystal device comprising a pair of substrates retaining a smectic liquid crystal therebetween and a plurality of bulkheads intersecting with a direction of a layer of the smectic liquid crystal provided on at least one of the pair of substrates,
wherein an elastic modulus E of the bulkheads, an outside pressure P, an area A1 of the substrate, a total area A2 of contact surfaces between the bulkheads and the substrate, and a volumetric shrinkage ratio xcex94Vlc/Vlc of the smectic liquid crystal within an ambient temperature range of the liquid crystal device satisfy the following relation:
(1/E)xc3x97Pxc3x97(A1/A2)xe2x89xa7xcex94Vlc/Vlc. 
According to a second aspect of the present invention, there is provided a liquid crystal device comprising a pair of substrates retaining a smectic liquid crystal therebetween and a plurality of stripe bulkheads intersecting with a direction of a layer of the smectic liquid crystal provided on at least one of the pair of substrates,
wherein an elastic modulus E, a height L, a spacing D, and a length H of the bulkheads, an outside pressure P, an area A1 of the substrate, a total area A2 of contact surfaces between the bulkheads and the substrate, and a volumetric shrinkage amount xcex94Vlc within an ambient temperature range of the liquid crystal device, of the smectic liquid crystal filled in a space defined by the pair of substrates and a pair of bulkheads satisfy the following relation:
(1/E)xc3x97Lxc3x97Pxc3x97(A1/A2)xe2x89xa7xcex94Vlc/(Dxc3x97H). 
According to a third aspect of the present invention, there is provided a method of producing a liquid crystal device, comprising in an order mentioned below the steps of:
(1) forming a stripe bulkhead on a first substrate;
(2) rubbing the first substrate substantially parallel to the direction of the stripe of the bulkhead;
(3) opposing and bonding the first substrate and a second substrate having no bulkhead formed thereon to each other, thereby forming a cell;
(4) filling the cell with a liquid crystal; and
(5) cooling the cell to a temperature not more than a smectic phase transition temperature of the liquid crystal, thereby forming a smectic layer substantially perpendicular to the bulkhead,
wherein an elastic modulus E of the bulkhead, an atmospheric pressure P, an area A1 of the second substrate, a total area A2 of contact surfaces between the bulkhead and the second substrate, and a volumetric shrinkage ratio Vlc/Vlc of the liquid crystal within a temperature variation range in the steps including and succeeding the step (4) satisfy the following relation:
(1/E)xc3x97Pxc3x97(A1/A2)xe2x89xa7xcex94Vlc/Vlc. 
When the liquid crystal device is constructed so as to satisfy the above relation, the bulkheads become able to shrink in response to the volumetric shrinkage of the liquid crystal, thereby preventing the occurrence of a void.